Isocyanate-functional reactive polyurethane adhesives (PU hot-melt adhesives) from prepolymers, which cure irreversibly by the action of moisture from the atmosphere or the materials glued to one another are known (EP-B 455 400). Such prepolymers are specific reaction products of polyester polyols and optionally polyether polyols with polyisocyanates. These reactive PU hot-melt adhesives are universally applicable for joining widely varying materials, such as e.g. plastic, glass, metal, leather or wood.
The setting times of PU hot-melt adhesives, i.e. the solidification times without the reaction of the components with one another starting, can be adjusted in the range from seconds up to minutes by modification of the recipes with components which are crystalline or amorphous at room temperature. In this context, the crystalline structures have the effect not only of a low melt viscosity and a rapid solidification after application, but also of a good low temperature elasticity due to the low glass transition temperature (DE-A 3 827 224, DE-A 4 114 220, EP-A 0 354 527).
In addition, components which are liquid at room temperature can also be used for modification of the properties. Thus, EP-A 0 340 906 describes reactive polyurethane hot-melts comprising a mixture of two polyurethane prepolymers, the first prepolymer being prepared from an amorphous polyol having a glass transition temperature of >20° C. and the second prepolymer being prepared from a polyol which is liquid at room temperature (Tg<20° C.).
The actual curing of reactive PU hot-melt adhesives, i.e. crosslinking reaction of the components with one another, takes place within some days by reaction of the isocyanate groups with water to give a thermosetting polyurea. Thereafter, the PU hot-melt adhesives are no longer meltable or e.g. soluble in solvents. Because of this, the cured adhesives have a good heat resistance and resistance to chemicals such as plasticizers, solvents, oils or fuels.
A disadvantage of these adhesives, however, is that because of their preparation process they have a high content of free monomeric polyisocyanates, such as e.g. 4,4′-diisocyanatodiphenylmethane (4,4′-MDI) or 2,4-diisocyanatotoluene or 2,6-diisocyanatotoluene (TDI). These monomeric polyisocyanates have a not inconsiderable vapour pressure at the application temperatures of the adhesives of about 130° C. to about 180° C. This means that the monomeric constituents escape into the environment in gaseous form. Because of these isocyanate emissions, corresponding industrial hygiene measures must be taken, such as e.g. installation of suitable extraction devices.
A further disadvantage of these adhesives is the evolution of carbon dioxide gas in the reaction with water to form a polyurea. As a result, the adhesive may foam in the glued joint and the desired position of the joint components may change.
In order to overcome the abovementioned disadvantages, silane-functional reactive hot-melt adhesives based on polyester polyols are described in the literature.
Thus, EP-A 0 202 491 discloses moisture-curing hot-melt adhesives which are obtainable in a first embodiment by reaction of polyester mixtures having solid polyester contents having glass transition temperatures above 10° C. and liquid polyesters having glass transition temperatures below −10° C. with an excess of polyisocyanates and subsequent reaction of the prepolymers obtained in this way, which contain free NCO groups, with amino- or mercaptosilanes. In a second embodiment, an adduct is first prepared from an amino- or mercaptosilane with a diisocyanate, the two components being reacted in the molar ratio of 1:1. The reaction of this addition product with the polyester mixture then takes place in a second step. The addition of catalysts to accelerate the curing reaction with moisture is not described. Nevertheless, dibutyltin dilaurate (DBTL) is employed as a catalyst for preparation of the hot-melt adhesives in all the embodiment examples. When the reaction has ended, the DBTL is not deactivated, and remains in the product, where in principle it can act as a curing catalyst. The hot-melt adhesives prepared in this way are said to already cure at room temperature with moisture from the atmosphere.
EP-A 0 354 472 describes alkoxysilane-terminated, moisture-crosslinking hot-melt adhesives obtainable by reaction of a) NCO-terminated silane compounds which can be prepared by reaction of amino- or mercaptosilanes with diisocyanates and linear alkylene diols having 2 to 12 carbon atoms and b) linear OH-— and/or NH2-terminated difunctional polymers obtainable by reaction of an excess of linear OH— and/or NH2-terminated polyesters, polyethers and/or polyurethanes with diisocyanates. For acceleration of the crosslinking reaction with moisture, the conventional acidic catalysts are mentioned, which are chosen e.g. from the group formed by tin(II) octoate, dibutyltin dilaurate, tetrabutyl titanate, zinc acetate and zinc acetylacetonate or the like.
Nevertheless, the hot-melt adhesives described in the two abovementioned laid-open specifications have acquired no industrial use to date, since because of their significantly reduced reactivity with moisture, compared with isocyanate end groups, the alkoxysilane end groups do not cure or cure only inadequately over a very long period of time without the addition of a catalyst, and therefore build up only inadequate strengths. If the Lewis acids mentioned are added as catalysts, the compositions lose their stability during storage in heat, since the catalysts mentioned also catalyse the transesterification of the polyester units present in the prepolymers with the low molecular weight alcohols, such as e.g. methanol or ethanol, split off from the alkoxy end groups. This leads to an irreversible degradation of the polymer chain and therefore to a destruction of the adhesive. Since the hot-melt adhesives are melted in heating ovens for application and are kept in the liquid state over a relatively long period of time, as a rule at least one working day, however, an adequate stability at high temperatures is necessarily required for industrial use.
The procedure described of preparing an adduct from diisocyanate and aminosilane in a first step represents a considerable industrial disadvantage, since the diadduct of two molecules of aminosilane and diisocyanate will also always form and expensive aminosilane is thus lost.
EP-A 0 480 363 describes a curable composition having at least two hydrolysable silyl groups obtainable by reaction of aliphatic polyesters which contain a hydroxyl and an acryloyl group in the molecule with an isocyanatosilane, subsequent reaction of the reaction product with an aminosilane, and subsequent reaction of the reaction product with a monofunctional isocyanate and/or with a polyisocyanate. Catalysts which are mentioned for curing of the composition are organotin compounds, such as e.g. dibutyltin dilaurate or tin octoate, acid compounds, such as e.g. p-toluenesulfonic acid or phosphoric acid esters, and amines, such as ethylenediamine, isophoronediamine or N,N-dimethyldodecylamine.
The abovementioned compositions are accessible only by an involved multi-stage preparation process and are therefore very expensive. Moreover, the polyesters having a hydroxyl and acryloyl group in the molecule required as the starting substance are not standard products such as are available in diverse forms e.g. for the preparation of reactive PU hot-melts, but are obtainable only with severe limitations. As a result, it is possible to only a limited extent to influence the properties of the hot-melt adhesives in a controlled manner by mixing of amorphous, liquid and crystalline polyesters having appropriate end groups. That which has already been stated above applies in respect of the storage stability in heat when the Lewis acid catalysts mentioned are used.
EP-A 0 096 250 discloses crosslinkable compositions which are liquid at temperatures below 100° C. and are based on polymers containing hydroxyl groups, in which only some of the hydroxyl groups are replaced by alkoxysilyl end groups. Particularly suitable polymers containing hydroxyl groups are, inter alia, polyester polyols. The preparation is carried out in a multi-stage synthesis. In a first embodiment, an adduct is first prepared from an amino- or mercaptosilane with a diisocyanate, the two components being reacted in the molar ratio of 1:1. In a second step, the reaction of this addition product with the polyol mixture in an amount such that the ratio of OH/NCO is less than 1:0.9 takes place. In a second embodiment, the reaction of an OH-containing polymer or a polymer mixture with an excess of diisocyanate takes place in the first step, with subsequent reaction of the prepolymers obtained in this way, which contain free NCO groups, with amino- or mercaptosilanes. In a third step, mixing of the resulting product of stage 2 with OH-containing polymers then takes place. In addition to compounds of tin and titanium, amines are also mentioned as suitable curing catalysts, but in the embodiment examples only DBTL is used. Nevertheless, these compositions are curable not only by access of moisture, but also by application of temperature.
This heat curing is a considerable industrial disadvantage, since the hot-melt adhesives are melted in heating ovens for application and are kept in the liquid state at elevated temperature for a relatively long period of time, as a rule at least one working day. However, these conditions lead to an at least partial crosslinking of the adhesive, as a result of which this becomes unusable. Furthermore, here also an involved multi-stage process for the preparation of the hot-melt adhesive disclosed is necessary. The adducts of aromatic isocyanates, such as e.g. MDI or TDI, and aminosilanes additionally are not stable to storage and must therefore be further reacted directly, which additionally makes the synthesis difficult. However, because of their considerably higher reactivity compared with the aliphatic diisocyanates, such aromatic diisocyanates are preferred for use in hot-melt adhesives.
WO 2004/005420 describes moisture-curing hot-melt adhesives which are prepared from a semicrystalline polyol, a substantially amorphous polyol having either branched primary OH groups or secondary OH groups or mixtures thereof, an aminosilane having secondary amino groups and an isocyanate, and for ecological reasons are substantially tin-free.
A method for the preparation of these moisture-curing hot-melt adhesives is moreover described. In this, in a first step a prepolymer is prepared from a semicrystalline polyol, a substantially amorphous polyol having either branched primary OH groups or secondary OH groups or mixtures thereof and an isocyanate. A tin-free catalyst can optionally be used in this step. 2,2′-Dimorpholinodiethyl ether (DMDEE) is mentioned as an example of a tin-free catalyst. In a second step, the prepolymer is then reacted with an aminosilane having a secondary amino group to give the moisture-curing hot-melt adhesive. In this case the two reaction steps should be carried out without the addition of a tin-containing catalyst. N-Alkyl-aminoalkyl-alkoxysilanes are used as aminosilanes having a secondary amino group. In addition, a catalyst which accelerates the curing with moisture can also be admixed to the finished hot-melt adhesive. Tertiary amines are mentioned by way of example. In the examples explaining the invention, DMDEE is used as a catalyst for the preparation of the prepolymers, i.e. in the reaction of the polyols with the isocyanate. However, no further curing catalyst is admixed to the finished hot-melt adhesive. Since the DMDEE remains in the hot-melt adhesive, however, it of course also acts as a catalyst in the curing with moisture. The examples and comparison example clearly show that if DMDEE is used, glued joints with an adequate final strength can only be achieved if the polyol mixture used for the preparation of the prepolymers comprises polyether polyols having secondary OH groups or amorphous polyesters which have branchings along the main chain. The various comparison examples (e.g. Comparison Example 2) show that with exclusive use of crystalline or unbranched polyester polyols, the DMDEE-catalysed hot-melt adhesives build up only completely inadequate final strengths, with values of ≦approx. 4 MPa after curing for 24 h. These values are significantly lower than the final strengths shown by the products comprising polyether polyols (approx. 9 to 12 MPa after curing for 24 h). The use of polyether polyols having secondary OH groups, however, is not desirable in all cases, since under certain circumstances they adversely influence the adhesive properties.
Sealants based on alkoxysilane-terminated polyether polyols have been known for a long time (EP-A 0 596 360 and WO 00/26271). In addition to organometallic curing catalysts, such as e.g. dibutyltin dilaurate, strongly basic bicyclic tertiary amines are also employed there as catalysts of the prior art. Thus, JP 08283366 describes 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) as particularly suitable. On the other hand, according to this laid-open specification other tertiary amines are not particularly suitable curing catalysts, since only extremely long curing times can be achieved with them. In particular, the compound bis-(N,N′-dimethylaminoethyl) ether (catalyst A-1) is mentioned as a negative comparison example and this catalyst is described as unsuitable for accelerating the curing reaction of the alkoxysilane-terminated polyurethane prepolymers with moisture.
Nevertheless, strongly basic bicyclic tertiary amines, such as 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) cannot be used as curing catalysts in the case of alkoxy-terminated hot-melt adhesives based on polyester polyols, since they lead to an irreversible degradation of the polyester units of the adhesive, and therefore to destruction thereof, under the action of heat, e.g. during the melting operation.
It can be seen from the prior art that there continues to be a need for polyester-based alkoxysilane-functional hot-melt adhesives which are stable to storage and cure very rapidly with moisture, and which have high strengths after complete curing with moisture.